Composite electroless coating containing particulate matter is a relatively new advancement in electroless (autocatalytic) plating. The subject of composite electroless coating with particulate matter appears to contradict earlier reports in the art of electroless plating, as well as some of the practices advocated by proprietary houses today.
Brenner, in U.S. Pat. Nos. 2,532,283 and 2,532,284, has described some of the basic concepts associated with electroless (autocatalytic) plating. In addition, Brenner and Riddell in Research, NBS 37, 1-4 (1946); Proc. Am. Electroplaters Soc., 33, 16 (1946); Research, NBS, 39, 385-95 (1947); and Proc. Am. Electroplaters Soc., 34, 156 (1947), have further discussed the electroless plating phenomenon and some of the precautions necessitated in effecting the process including awareness of the detrimental effect(s) associated with the presence of finely divided particles.
Gutzeit et al and Talmey et al in U.S. Pat. Nos. 2,819,187 and 2,658,839 have noted with great detail the sensitivity of electroless plating to homogeneous decomposition, some of which is caused by the presence of a solid insoluble phase.
U.S. Pat. Nos. 2,762,723 and 2,884,344 show some typical electroless plating stabilizers from the prior art used in the prevention of homogeneous decomposition. U.S. Pat. No. 3,234,031 shows some further electroless plating stabilizers of the prior art. A general review of conventional electroless plating stabilizers is noted in G. Salvago et al, Plating, 59, 665 (1972). The fundamental importance of the concentration of the electroless plating stabilizers used in the prior art is noted in Feldstein et al, J. Anal. Chem., 42, 945 (1970); Feldstein et al, J. Electrochem. Soc., 118, 869 (1971); Feldstein et al, J. Anal. Chem. 43, 1133 (1971); Feldstein et al, J. Electrochem. Soc., 117, 1110 (1970). In Electroless Nickel Newsletter, Edition II, Sep. 1980, in describing composite coatings the author concluded his survey: "Most conventional electroless plating baths are not well suited to composite plating, as the stabilizer is affected by the high concentration of particulate matter." The above publications and patents are incorporated herein by reference.
The previous findings stem from the recognition by those skilled in the art that electroless plating compositions are generally chemical systems which are thermodynamically unstable. Hence, any contamination may lead to the bulk of decomposition of the bath. Even at the present time, many commercially available proprietary electroless plating baths recommend that a mechanical filtration (through a 3 micron filter) should be incorporated to insure the maintainance of cleanliness in the electroless plating bath from insoluble foreign matter.
Despite previous findings it is now recognized that a wide variety of particulate matter may be incorporated in the electroless plating bath leading to the codeposition of the particulate matter along with the metallic or alloy matrix. In a German patent application No. B90776, included herein by reference, Metzger et al suggested the incorporation of insoluble particulate matter into the electroless plating bath to lead to composite coating. Though Metzger et al specified several plating baths of nickel, copper, and cobalt, there were no actual examples provided showing the codeposition and stability of such composite plating baths. Nevertheless, U.S. Pat. Nos. 3,617,363 and 3,753,667 were issued based upon the German application.
The following publication and the references therein are further provided: Electroless Nickel Coatings-Diamond Containing, R. Barras et al, Electroless Nickel Conference, Nov. (1979) Cincinatti, Ohio or N. Feldstein et al, Product Finishing July (1980) p. 65. They are included herein by reference.
In general it is noted that the electroless plating bath contains a metal salt as a source of the metal for the reduction, a complexing agent, a suitable reducing agent, a pH adjuster, and a stabilizer. Some prior art stabilizers are noted in the above cited publications and patents. The prior art stabilizers are known to act as "poisoning agents" of the catalytic sites.
For further appreciation of the state of the art a comprehensive review is noted by F. Pearlstein, Chapter 31 in "Modern Electroplating", 3rd Edition, Frederick A. Lowenheim editor 1974, John Wiley and Sons, Inc., publisher, which is included herein by reference. In Table I of this chapter typical composition(s) is noted both for acidic and alkaline type baths. The generic components of the bath include a nickel salt, sodium hypophosphite, a complexing agent, a pH modifier component, and a stabilizer (e.g., lead ions). The author notes that the formation of insoluble nickel phosphite interferes with the chemical balance of the solution by the removal of nickel ions, and has a detrimental effect on the quality of the deposit, and may also trigger spontaneous bath decomposition.
Regardless of previously encountered problems, in composite electroless plating baths the particulate matter which is being added, e.g., 5 micron of silicon carbide, has a surface area of about 2 meters.sup.2 /gram. The surface area is generally increased with decreased particle size. In fact, the surface area for the particulate matter contemplated in composite coatings and the present invention is greater than the recommended work load for plating. Pearlstein, in the above cited chapter (p. 718), notes that the bath's stability is adversely affected by excessive loads, and he suggests a limit of about 125 cm.sup.2 /l.
By contrast, an electroless plating bath with a few grams (e.g., 5 g/l) of finely divided particulate matter may result in an added surface area in the range of 100,000 cm.sup.2 /l which is significantly greater than the suggested load limit per plating volume solution.
From these semi-quantitative analyses the danger of adding the finely divided particulate matter is recognized. In fact, in conventional electroless plating continuous or semi-continuous filtration is recommended to remove finely divided matter. In addition, from the above reviewed state of the art, it is recognized that it is highly impractical to stabilize composite baths by the incorporation of extra stabilizer(s), (e.g., lead ions, thiourea, etc.). The addition of any significant extra stabilizer(s), though it may lead to bath stabilization, will also reduce significantly the plating value(s) to lower and impractical values.
Though composite coating by electroless plating is well documented in the above cited patents and publications, nevertheless there still remains major concern with the introduction of finely divided particulate matter having a high surface area. Yet, based on the above references, there does not appear to have been an effort toward the development of special baths with would serve the particular needs of composite electroless coatings.
It is thus the general and overall objective of the present invention to provide improved electroless plating baths particularly suitable for composite coatings which will provide longer viability as well as improved coating.